Quinoidal Molecules Research Articles

Thieno[3,2-b]thiophene-diketopyrrolopyrrole-based quinoidal small molecules: synthesis, characterization, redox behavior, and n-channel organic field-effect transistors.

We report the synthesis, characterization, redox behavior, and n-channel organic field-effect (OFET) characteristics of a new class of thieno[3,2-b]thiophene-diketopyrrolopyrrole-based quinoidal small molecules 3 and 4. Under ambient atmosphere, solution-processed thin-film transistors based on 3 and 4 exhibit maximum electron mobilities up to 0.22 and 0.16 cm(2) V(-1) s(-1) , respectively, with on-off current ratios (Ion /Ioff ) of more than than 10(6) . Cyclic voltammetry analysis showed that this class of quinoidal derivatives exhibited excellent reversible two-stage reduction behavior. This property was further investigated by a stepwise reductive titration of 4, in which sequential reduction to the radical anion and then the dianion were observed.

Heterohexacene Diimides: Anti- and Syn- Isomers and Quinonoid Forms

A number of anti- and syn- isomers of heterocyclic hexacene diimides containing NH and O/S are synthesized. Two stable quinonoid diimides displaying low LUMO levels at less than -4.1 eV are obtained via oxidation of the anti- isomers. Reducing the isolated quinoidal molecules back to dihydro- forms offer pure anti- isomers.

Theoretical study on the transport properties of oligothiophene–diketopyrrolopyrrole derivatives: quinoidal versus aromatic

Density functional theory calculations were carried out to investigate the transport mechanism of three oligothiophene–diketopyrrolopyrrole (T-DPP) derivatives. The results show that quinoidal molecule 1 that has dicyanovinyl has lower the lowest unoccupied molecular orbital (LUMO) energy level and larger adiabatic electron affinity (EAa) comparing with quinoidal 2 that has monocyanovinyl as terminal groups, signifying that to increase the extent of quinoid could improve the stability of the molecules in the air. Moreover, quinoidal molecules 1 and 2 have lower LUMO energy level, larger EAa, and electron-transfer integral (Ve) comparing than aromatic molecule 3, which indicates that quinoidal T-DPP derivatives are more in favor of electron than hole transport compared with their aromatic ones. Thus, constructing quinoidal architecture was also an effective way to design n-type transport materials besides the conventional idea that introducing electron-withdrawing groups.

Theoretical studies on the quinoidal thiophene based dyes for dye sensitized solar cell and NLO applications.

A series of quinoidal thiophene based dye molecules were designed and their optoelectronic properties were studied for dye sensitized solar cell (DSSC) applications. The efficiency of the designed dye molecules was analyzed using various parameters such as the HOMO-LUMO energy gap, absorption spectra, light harvesting efficiency (LHE), exciton biding energy (Eb) and free energy change for electron injection (ΔG(inject)). The simulated absorption spectra of the quinoidal thiophene molecules show that the electron withdrawing group substituted molecules exhibit dual band characteristics. We found that the cyano-[5'-(4″-amino benzylidene)-5H-thiophen-2'-ylidene] acetic acid based molecules, QT2B, QT4B, QT5 and QT6, are good candidates for DSSC applications. Furthermore, the study on the polarizability and hyperpolarizability of the designed molecules showed that the electron withdrawing group substituted QT2B-X molecules (X = Cl, Br, CF3, CN and NO2) are good candidates for NLO applications.

Near-infrared absorbing heterocyclic quinoid donors for organic solar cell devices

New heterocyclic quinoid donor molecules were designed and synthesized for application to organic solar cells in the near-infrared region. Devices using one of these quinoid molecules as a photoexcitable donor and C60 as an acceptor function in the visible and the near-infrared regions up to 890 nm.

Synthesis of quinoidal molecules: Strategies towards bioactive compounds with an emphasis on lapachones

Naphthoquinoidal compounds are of great interest in medicinal chemistry. In recent years, several synthetic routes have been developed to obtain bioactive molecules derived from lapachones. In this mini-review, we focus on the synthetic aspects and strategies used to design these compounds and on the biological activities of these substances for the development of drugs against the neglected diseases leishmaniasis and Chagas disease as well as malaria, tuberculosis and cancer. Three strategies used to develop bioactive naphthoquinoidal compounds are discussed: (i) C-ring modification, (ii) redox centre modification and (iii) A-ring modification. Among these strategies, reactions such as copper-catalysed azide–alkyne cycloaddition (click chemistry), palladium-catalysed cross couplings, and heterocyclisations will be discussed for the development of naphthoquinoidal compounds against Trypanosoma cruzi, Leishmania and cancer. The aim of derivatisation is the generation of novel molecules that inhibit cellular organelles/processes, generate reactive oxygen species (ROS) and increase lipophilicity to enhance penetration through the plasma membrane. Modified lapachones have emerged as promising prototypes for the development of drugs against neglected diseases and cancer.

A solution-processable dicyano-substituted quinoidal oligothiophene for air-stable ambipolar organic field-effect transistors

A novel solution-processable dicyano-substituted quinoidal oligothiophene molecule CDT-2T(CN)4 was synthesized and characterized. CDT-2T(CN)4 displayed high thermal stability and broad thin film absorption in the near IR region. Electrochemical results showed that CDT-2T(CN)4 exhibited reversible oxidation and reduction peaks with the LUMO energy level at −4.2 eV and the HOMO energy level at −5.3 eV. The solution processed thin film transistors of CDT-2T(CN)4 displayed electron-dominant ambipolar transport behavior under ambient conditions. The optimal saturation hole and electron mobilities were 0.003 and 0.023 cm2 V−1 s−1, respectively, one of the highest values for solution processed quinoidal based ambipolar organic semiconductors with good ambient stability.

Diketopyrrolopyrrole-Containing Quinoidal Small Molecules for High-Performance, Air-Stable, and Solution-Processable n-Channel Organic Field-Effect Transistors

We report the synthesis, characterization, and application of a novel series of diketopyrrolopyrrole (DPP)-containing quinoidal small molecules as highly efficient n-type organic semiconductors in thin film transistors (TFTs). The first two representatives of these species exhibit maximum electron mobility up to 0.55 cm(2) V(-1) s(-1) with current on/current off (I(on)/I(off)) values of 10(6) for 1 by vapor evaporation, and 0.35 cm(2) V(-1) s(-1) with I(on)/I(off) values of 10(5)-10(6) for 2 by solution process in air, which is the first demonstration of DPP-based small molecules offering only electron transport characteristics in TFT devices. The results indicate that incorporation of a DPP moiety to construct quinoidal architecture is an effective approach to enhance the charge-transport capability.

The electronic structure of p-xylylene and its reactivity with vinyl molecules

The electronic states of p-xylylene molecule were described at the multi-configurational CASSCF/MRMP2 level of theory. The closed-shell singlet state representing the quinoidal p-xylylene molecule was predicted to be the ground electronic state whereas the triplet (benzoidal) and the singlet open-shell states were found to be much higher in energy (by 159 and 423 kJ/mol, respectively, as found at the CASSCF(8,8)/6-31+G(d) level). The reactions of p-xylylene with variously substituted vinyl molecules (i.e., CH 2 CH 2, CH 2 CHCl, CH 2 CHCN, CH 2 CHNO 2, CH 2 CHCF 3, CH 2 CHC 6H 5, CH 2 CHCOOC 6F 5, and CH 2 C(CN) 2) were investigated with the DFT (using the B3LYP and MPW1 K functionals) and MP2 methods using the 6-31++G(d,p) basis set and the activation barriers in the 33–113 kJ/mol range were predicted. Since some of the activation barriers found are smaller than the energy required to initialize the parylene polymerization (63 kJ/mol) the reactions of the p-xylylene with vinyl molecules is postulated to be competitive with the polymerization process.

Towards the Efficiency of Pharmacologically Active Quinoid Compounds: Electron Transfer and Formation of Reactive Oxygen Species

In this review article, the structure, properties, stability and biological application of redox-active quinones are presented. A series of quinoid molecules is evaluated in terms of their ability to act as electron-transfer active compounds using cyclovoltammetric, electron paramagnetic resonance (EPR) and spin-trapping techniques. Redox potentials and electron distribution of the intermediate radical anions are shown to be decisive factors for the generation of reactive oxygen species (ROS). Mechanisms of ROS generation in dark by biological electron-transfer reaction or under photoexcitation have been proposed and experimentally verified. For site-specific damage of tumors, some quinone derivatives were covalently bound with the luteinizing hormone-releasing hormone (LH–RH or GnRH) that produces specific complexes with receptors on the surface of cancer cells. The properties of obtained conjugates to be bound with the different lines of cancer cells (αT3-1, M2R, LNCaP) were tested. EPR was used for the estimation of efficacy of ROS production by the conjugates in solution and in the complex with cancer cells. The toxicity of these conjugates as well as their stability in the stimulated oxidative stress were tested. The proposed approach could be useful in creating a new family of addressed anticancer drugs, including compounds for the treatment of tumors by photodynamic therapy.

A planar organic near infrared light detector based on bulk heterojunction of a heteroquaterphenoquinone and poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1, 4-phenylene vinylene

Planar organic detectors for the near infrared region of the light spectrum have been realized, processing from solution the quinoid molecule 5,5′-bis(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadiene-1-ylidene)-5,5′-dihydro-2,2′-bithiophene (QBT), which shows a peculiar absorption at 680 nm. Sensitization of the active material was obtained by creating a bulk donor-acceptor interface with poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene], which acts as hole acceptor. Photoexcitation and relaxation dynamics in pristine materials and bulk heterojunction have been investigated by means of pump-probe measurements, revealing charge transfer from QBT to polymer and efficient energy transfer the other way round.

Classification of Dynamic Vibronic Couplings in Vibrational Real-Time Spectra of a Thiophene Derivative by Few-Cycle Pulses

Pump-probe spectroscopy was performed with a few cycle pulses of 6.7 fs duration. The electronic transition intensity modulation was induced by molecular vibration in a quinoid thiophene molecule in solution. The real-time vibrational features were analyzed in terms of dependence of vibrational amplitude and phase on probe photon energy. The electronic transition probability is modulated by molecular vibration via vibronic coupling. Changes in the spectral shape and intensity of the time-resolved spectrum were studied by tracking characteristic spectral features including the peak frequency and intensity, spectral bandwidth, and band-integrated intensity. From the analysis the modulation mechanisms were classified into two groups: (1) Condon type and (2) non-Condon type. The features of the wave packet motions were also classified into zeroth-order derivatives due to quasi-pure non-Condon type and first- and second-order derivative types due to the displacement of the potential minimum and the potential curvature change associated with the relevant vibronic transition, respectively.

Confined breather-type excitation in a quinoidal thiophene after sub-5fs pulse excitation

A quinoidal thiophene molecule which is a prototype of the repeated unit of s-trans-cis-polyacetylene was studied by sub-5 fs spectroscopy. A breatherlike mode modulation of both the amplitude and frequency of the C[Double Bond]C stretching was observed for the first time. It generates two sidebands of 2238 and 700 cm(-1) with a separation of 769 cm(-1) corresponding to the modulation frequency. The latter mode is very close to the C-S-C ring deformation, which indicate that this mode mediates coupling between the breather mode and the C(beta)-C(beta) stretching mode. It was also found that the breather-type excitation has a longer lifetime (2-3 ps) than in all-trans-polyacetylene (50 fs) due to its confinement.

Synthesis and Supramolecular Properties of Molecular Clips with Anthracene Sidewalls

Novel molecular clips with anthracene sidewalls (1 a-c) were synthesized; they form stable host-guest complexes with a variety of electron-deficient aromatic and quinoid molecules. According to single-crystal structure analyses of clip 1 c and 1,2,4,5-tetracyanobenzene (TCNB) complex 14@1 b, the clips' anthracene sidewalls have to be compressed substantially during the complex formation to provide attractive pi-pi interactions between the aromatic guest molecule and the two anthracene sidewalls in the complex. The compression and expansion of aromatic sidewalls are calculated by molecular mechanics to be low-energy processes, so the energy required for compression of the anthracene sidewalls during complex formation is apparently overcompensated by the gain in energy resulting from the attractive pi-pi interactions. The finding that complexes of the clips 1 a-c are more stable than those of the corresponding clips 2 a-c can be explained in terms of the larger van der Waals contact surfaces of the anthracene sidewalls in 1 a-c (relative to the naphthalene sidewalls in 2 a-c). Color changes resulting from charge-transfer (CT) bands are observed in complex formation by 1 a-c: from colorless to red or purple with TCNB (14), and from yellow to green with 2,4,7-trinitro-9-fluorenone TNF (17). Independently, the host 1 b and guest 14 fluoresce from their respective excited singlet states, whilst in the complex 14@1 b the charge-transfer state quenches the higher-energy singlet states of the two components, and as a result luminescence is only observed from this new CT state. To the best of our knowledge, complex 14@1 b is the first example of CT luminescence from a host-guest complex. The binding constant determined for the formation of the TCNB complex 14@1 b from a UV/Vis titration experiment (Ka = 12 400 m(-1)) agrees well with the value (K(a) = 12 800 m(-1)) obtained by 1H NMR titration.

Charge separation in ground-state 1,2,4,5-tetra-substituted benzene derivatives.

The conditions required for a formal biradical to exist in a zwitterionic form in the ground state are discussed following the recent experimental observation of zwitterionic structure in the ground state of a quinoid molecule (di-tert-butyl derivative of 2,5-diamino-1,4-benzoquinonediimine, I). A unique characteristic of molecules of this class is the fact that they may be considered as being formed by the union of two radicals, each having an odd number of pi electrons. In the case of I, one fragment carries the two amino group having 7 pi electrons; it acts as the electron donor. The other fragment carries the two oxygen atoms (carrying 5 pi electrons) and acts as an electron acceptor. A model that predicts the properties of these systems is presented, based on previous work on non-Kekule hydrocarbons(2,3) and on the electron donating and attracting properties of the donor and acceptor groups, respectively. The zwitterion is formed by an electron transfer leading to two subunits carrying 6 pi electrons each and may become more stable than the triplet biradical even in the gas phase (i.e., in the absence of an external field) if the ionization potential of the donor is small (of the order of 3-4 eV). In some cases solvation in a polar solvent is required to make the zwitterionic form the lowest energy species on the ground-state surface. The 'spacer' between the donor and acceptor groups (which need not be necessarily derived from an aromatic structure) can be varied and influences the overall dipole moment that is calculated in some cases to be quite large (over 20 D in the gas phase).

Spectroscopic and theoretical study of the molecular and electronic structures of a terthiophene-based quinodimethane.

The UV/Vis, infrared absorption, and Raman scattering spectra of 3',4'-dibutyl-5,5"-bis(dicyanomethylene)-5,5"-dihydro-2,2':5',2"-terthiophene have been analyzed with the aid of density functional theory calculations. The compound exhibits a quinoid structure in its ground electronic state and presents an intramolecular charge transfer from the terthiophene moiety to the C(CN)2 groups. The molecular system therefore consists of an electron-deficient terthiophene backbone end-capped with electron-rich C(CN)2 groups. The molecule is characterized by a strong absorption in the red, due to the HOMO-->LUMO pi-pi* electronic transition of the terthiophene backbone that shifts hypsochromically on passing from the solid state to solution and with the polarity of the solvent. The analysis of the vibrational spectra confirms the structural conclusions and supports the existence of an intramolecular charge transfer. Vibrational spectra in several solvents and as a function of temperature have also been studied. Significant frequency upshifts of the vibrations involved in the pi-electron-conjugated pathway have been noticed upon solution in polar solvents and with the lowering of the temperature. Finally, we propose a quinoid molecule as a reliable structural and electronic model for dication species in doped oligothiophenes or for bipolaron charged defects in doped polythiophene.

Optical properties of a quinoid molecule

We report on linear and third order nonlinear optical properties of the quinoid molecule 5,5 ′-bis(3,5-di- tert-butyl-4-oxo-2,5-cyclohexadiene-1-ylidene)-5,5 ′-dihydro-2, 2 ′-bithiophene.Variable angle spectroscopic ellipsometry has been used to determine the dispersion of the dielectric in a spectral range from the near infrared to the near ultraviolet. We have measured a high value of the index of refraction in the transparency region (i.e., n=1.886 at λ=1.5 μ m), indicating the high polarisability of the molecule under study. The nonlinear optical properties have been measured by third harmonic generation in the near infrared spectral range. At λ=1.5 μ m the nonlinear coefficient of the molecule has been estimated to be 4.2 × 10 −19 m 2/V 2 (3.0 × 10 −11 esu).

Enhancement of PMMA nonlinear optical properties by means of a quinoid molecule

Abstract In this paper we present the study of the linear and nonlinear optical properties of organic films based on mixing a polymeric matrix (PMMA) and a highly nonlinear organic material recently synthesised and characterised, namely (5,5 ′ -Bis(3,5-di- tert -butyl-4-oxo-2,5-cyclohexadien-1-ylidene)-5,5 ′ -dihydro-2,2 ′ -bithiophene), with the prospective of reaching a good compromise between good processability and high nonlinear optical (NLO) properties of the solid solution. Linear optical characterisation is performed by means of variable angle spectroscopic ellipsometry in the spectral range between 260 and 1700 nm. Third order NLO properties of the samples have been investigated by third harmonic generation at fundamental wavelength of 1500 nm. Different guest/host concentrations have been studied. Due to the high solubility of the guest into the polymeric matrix, we have been able to reach concentration up to 50% in weight, reaching an enhancement of the NLO properties of PMMA by three order of magnitudes.

Electrochemical reduction of indigo in fixed and fluidized beds of graphite granules

Reducing agents required in the dyeing process for vat and sulfur dyes cannot be recycled and lead to problematic waste products. The electrochemical reduction of indigo on a fixed bed cathode consisting of graphite granules has been investigated by spectrophotometric experiments in laboratory cells. Experiments yield information about the kinetics and show the possibility of this process for production of water soluble leuco indigo, which offers environmental benefits. The influence of noble metals deposited on the granules and of different pretreatment methods of the graphite is demonstrated. In addition, the immobilization of quinoid molecules on the graphite surface has been investigated.

Polarizability and hyperpolarizability of push–pull quinoid molecules Simulation of donor–acceptor pairs

AM1 calculations are performed to study the non-linear optical (NLO) properties of push–pull quinones. The variations of molecular structure, linear (α) and NLO polarizabilities (β, γ) for a set of push–pull quinones are discussed in terms of quinoid–benzenoid character (QBC), and the strength of donor–acceptor pairs (δDA). The parameter δDA, already used for the polyenebridge, is still valid in rationalizing the donor–acceptor strength for quinoid derivatives. In addition, we examined how an external electric field created by ‘Sparkles’ in the MOPAC package drives a quinoid molecule from the quinoid limit to the benzenoid limit, and how it affects the NLO properties. The results show that the effect produced by Sparkles is qualitatively similar to that of donor–acceptor pairs.